Combined effects of particle size and humic acid corona on the aggregation kinetics of nanoplastics in aquatic environments.

Nanoplastics (NPs) contaminant in aquatic environments is one of the pressing environmental concerns globally. However, the combined effects of particle size and humic acid (HA) corona on the aggregation behavior of NPs have not been revealed yet. Therefore, this study explored the influence of HA corona on the aggregation kinetics of NPs with three different particle sizes under various water quality conditions. Results showed that in the absence of HA corona, the aggregation kinetic processes of all the three NPs were affected by the repulsive force originating from the hydration layer. Moreover, the smaller the particle size, the more obvious the effect. HA corona played a steric hindrance role for all the three NPs based on the extended-Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory in monovalent solutions, resulting in the impediment of aggregation. Whereas, in divalent solutions, the HA corona of 100 and 200 nm NPs experienced three stages: deformation, electrostatic-patch and bridging; while that of 40 nm NPs underwent electrostatic-patch and steric hindrance. The larger number of HA molecules distributed on 100 and 200 NPs surfaces led to more interactions with Ca and NPs, which was the key factor for HA corona to play more diverse roles. According to the two dimension correlation spectroscopy analysis (2D-COS), the structural change in the interaction between HA and NPs was that the aromatic ring of NPs took precedence, followed by the carbonyl groups of HA. This study provided new insights into the combined effects of HA corona and particle size on the aggregation kinetics of NPs and established a theoretical foundation for predicting and assessing the transport and fate of NPs.